Two isoproteins of metallothioneins were purified to homogeneity from liver of 100 rats that had been pretreated with Cd ions. Heavy metal ions, such as Cu ions, Zn ions, Hg ions, Ag ions and Pb ions were selected to replace Cd ions in purified metallothioneins to form different thionein-metal complexes. Proton and C13 NMR spectroscopy were employed to investigate the apparent structure differences of different thionein-metal complexes in an attempt to understand their physiological roles as metal binding proteins.